Research On Peak-to-Average Power Ratio Reduction Technology In OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is considered as mainstream technology of the fourth generation mobile communication due to high spectral efficiency and the robustness against multi-path delay spread. However, one major drawback of OFDM is the high peak-to-average power ratio (PAPR) of the time-domain signals. Nonlinear distortion may be introduced when the OFDM signals with high PAPR pass through High Power Amplifier (HPA), which leads to deterioration of system performance. Although increasing the linear range of HPA can mitigate this problem, it will increase the system cost and reduce the efficiency of HPA. Therefore, how to effectively reduce the PAPR of OFDM signals is one research focus of communication. This thesis further studies PAPR problem of OFDM systems and proposes effective solutions and circuit structures.Firstly, partial transmit sequence (PTS) method is studied. PTS method has three sub-block partitioning schemes, namely, interleaved partitioning (IP), adjacent partitioning (AP) or random partitioning (RP). IP-PTS method has the lowest complextiy, but its PAPR performance is inferior to that of other two partitioning schemes. For this problem, this thesis optimizates phase factor vector in IP-PTS method and reduce search range of phase factor without loss of PAPR performance. To improve PAPR performance of IP-PTS method, we propose conjugate IP-PTS method based on independent phase factor vector. Using complex conjugate property of Discrete Fourier Transform (DFT) and the characteristics of conjugate pairs in phase factor vector set, we optimize conjugate IP-PTS method to reduce its complexity. We propose PTS method in which real and imaginary parts are multiplied separately with phase factors, moreover, PAPR is jointly optimized in real part and imaginary part.Secondly, time-domain IP-PTS method is studied. Using the circular convolution property of DFT, time-domain IP-PTS method only needs to calculate one IFFT. However, the search and combination of phase factors still need higher computational complexity. In order to improve this problem, this thesis proves that the phase factor vectors in time-domain IP-PTS method are not independent and proposes the optimization scheme of phase factor vector. We research the combination characteristic of phase factors and propose three schemes respectively based on storage-unit, pipeline and select-path to implement the combination of phase factors.Subsequently, the companding method is studied. Companing is a low-complexity method for reducing PAPR of OFDM signals. Using inverse companding transform in the receiveer, this method can recover original data and effectively reduce the nonlinear distortion. Research focus on this method is to propose companding transform function with better performance. For real and complex OFDM signals, this thesis presents companding functions based on triangular distribution. Through implementing companding transform on OFDM signals, the distribution of companded signals approximate the target distribution with low PAPR. According to the statistical distribution of the original signals and the target distribution of companded signals, we derive function expressions of companding transforms. Moreover, this thesis researches the performance of the companded systems, including PAPR, BER and power spectral density.Finally, circuit design of PTS method is studied. PTS method suffers from high complexity because of computation of multiple IFFTs. For this issue, this thesis proposes four IFFT implementation schemes, namely, multi-path scheme, improved multi-path scheme, the scheme using Cookey-Tukey FFT algorithm and the scheme using only an N point IFFT. We design two kinds of PAPR optimization module structure, namely, serial and parallel structures. For PTS method, we complete the circuit design and synthesis using SMIC 0.18μm CMOS standard cell library.